Heat-resistant magnesium alloy for gravity casting with high creep resistance

ABSTRACT

The present invention provides a heat-resistant magnesium alloy for gravity casting, which has excellent high-temperature resistance, tensile strength, and creep resistance properties. 
     In preferred embodiments, the present invention provides a heat-resistant magnesium alloy for gravity casting with high creep resistance, the magnesium alloy containing magnesium as a main component, 1.00 to 3.00 wt % neodymium, 2.00 to 6.00 wt % misch metal, 0.10 to 1.00 wt % zinc, 0.10 to 1.00 wt % zirconium, 0.01 to 0.50 wt % yttrium, 0.01 to 0.10 wt % calcium, 0.008 wt % silicon, 0.004 wt % iron, 0.003 wt % copper, and 0.001 wt % nickel.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims under 35 U.S.C. §119(a) the benefit of Korean Patent Application No. 10-2010-0050432 filed May 28, 2010, the entire contents of which are incorporated herein by reference.

BACKGROUND

(a) Technical Field

The present disclosure relates to a heat-resistant magnesium alloy for gravity casting with high creep resistance. More particularly, it relates to a heat-resistant magnesium alloy for gravity casting, which has excellent high-temperature resistance, tensile strength, and creep resistance properties.

(b) Background Art

Magnesium alloys have an excellent specific strength, a high elastic modulus, a high ability to absorb vibration, shock, and electromagnetic waves, and thus they are widely used to reduce the weight of various transportation equipment including vehicles.

In particular, the possibility of using heat-resistant magnesium alloys, which have improved heat-resistant properties of magnesium alloys at room temperature, is increased so as to reduce the weight of power trains for vehicles.

The heat-resistant magnesium alloys can be classified into die casting alloys such as, but not limited to, AJ (Mg—Al—Sr) alloy, AE (Mg—Al—Re) alloy, and MRI153 (Mg—Al—Sr—Ca—Re) alloy, and gravity casting alloys such as, but not limited to, ZE (Mg—Zn—Zr—Re) alloy, ZK (Mg—Zn—Zr) alloy, and MRI202S (Mg—Nd—Zn—Zr—Y—Ca) alloy.

In the above-described gravity casting alloys, the MRI202S alloy is recognized as a material which has the highest creep resistance among the existing heat-resistant magnesium alloys, and thus it is applicable to a cylinder block material of an engine which operates at an ambient temperature of 175 C.°.

However, the MRI202S alloy is expensive, and Korea depends entirely on imports for its supplies, and thus it is difficult to use it as a component material such as an engine cylinder block.

Therefore, there remains a need in the art for the development of magnesium alloy materials which are more inexpensive and have higher heat resistance.

The above information disclosed in this the Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.

SUMMARY OF THE DISCLOSURE

In one aspect, the present invention provides a heat-resistant magnesium alloy for gravity casting with high creep resistance, which can suitably improve high-temperature tensile strength and creep resistance properties by controlling the content of the components of a conventional MRI202S alloy and, at the same time, by further adding misch metal as a rare earth metal, and can improve the hardness of the magnesium alloy by controlling the age hardening temperature and time.

In a preferred embodiment, the present invention provides a heat-resistant magnesium alloy for gravity casting with high creep resistance, the magnesium alloy containing magnesium as a main component, 1.00 to 3.00 wt % neodymium, 2.00 to 6.00 wt % misch metal, 0.10 to 1.00 wt % zinc, 0.10 to 1.00 wt % zirconium, 0.01 to 0.50 wt % yttrium, and 0.01 to 0.10 wt % calcium, 0.008 wt % silicon, 0.004 wt % iron, 0.003 wt % copper, and 0.001 wt % nickel.

In another preferred embodiment, the magnesium alloy may contain magnesium as a main component, 1.5 to 2.50 wt % neodymium, 2.00 to 4.00 wt % misch metal, 0.30 to 0.60 wt % zinc, 0.20 to 0.50 wt % zirconium, 0.09 to 0.20 wt % yttrium, and 0.01 to 0.05 wt % calcium, 0.008 wt % silicon, 0.004 wt % iron, 0.003 wt % copper, and 0.001 wt % nickel.

Other aspects and preferred embodiments of the invention are discussed infra.

It is understood that the term “vehicle” or “vehicular” or other similar term as used herein is inclusive of motor vehicles in general such as passenger automobiles including sports utility vehicles (SUV), buses, trucks, various commercial vehicles, watercraft including a variety of boats and ships, aircraft, and the like, and includes hybrid vehicles, electric vehicles, plug-in hybrid electric vehicles, hydrogen-powered vehicles and other alternative fuel vehicles (e.g. fuels derived from resources other than petroleum). As referred to herein, a hybrid vehicle is a vehicle that has two or more sources of power, for example both gasoline-powered and electric-powered vehicles.

The above features and advantages of the present invention will be apparent from or are set forth in more detail in the accompanying drawings, which are incorporated in and form a part of this specification, and the following Detailed Description, which together serve to explain by way of example the principles of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other features of the present invention will now be described in detail with reference to certain exemplary embodiments thereof illustrated the accompanying drawings which are given hereinbelow by way of illustration only, and thus are not limitative of the present invention, and wherein:

FIGS. 1 and 2 are graphs showing the results of age-hardening behavior test on a conventional magnesium alloy and a magnesium alloy in accordance with an exemplary embodiment of the present invention, respectively.

FIG. 3 is a scanning electron microscope (SEM) image of a magnesium alloy in accordance with an exemplary embodiment of the present invention.

FIG. 4 is a graph showing the results of room-temperature yield strength test on a magnesium alloy in accordance with an exemplary embodiment of the present invention and a conventional magnesium alloy, respectively.

FIG. 5 is a graph showing the results of high-temperature yield strength test on a magnesium alloy in accordance with an exemplary embodiment of the present invention and a conventional magnesium alloy, respectively.

FIG. 6 is a graph showing the results of creep resistance test on a conventional magnesium alloy.

FIG. 7 is a graph showing the results of creep resistance test on a magnesium alloy in accordance with an exemplary embodiment of the present invention.

It should be understood that the appended drawings are not necessarily to scale, presenting a somewhat simplified representation of various preferred features illustrative of the basic principles of the invention. The specific design features of the present invention as disclosed herein, including, for example, specific dimensions, orientations, locations, and shapes will be determined in part by the particular intended application and use environment.

DETAILED DESCRIPTION

As described herein, the present invention features a heat-resistant magnesium alloy for gravity casting with high creep resistance, the magnesium alloy containing magnesium as a main component, 1.00 to 3.00 wt % neodymium, 2.00 to 6.00 wt % misch metal, 0.10 to 1.00 wt % zinc, 0.10 to 1.00 wt % zirconium, 0.01 to 0.50 wt % yttrium, 0.01 to 0.10 wt % calcium, 0.008 wt % silicon, 0.004 wt % iron, 0.003 wt % copper, and 0.001 wt % nickel.

In one embodiment, the magnesium alloy contains magnesium as a main component, 1.5 to 2.50 wt % neodymium, 2.00 to 4.00 wt % misch metal, 0.30 to 0.60 wt % zinc, 0.20 to 0.50 wt % zirconium, 0.09 to 0.20 wt % yttrium, 0.01 to 0.05 wt % calcium, 0.008 wt % silicon, 0.004 wt % iron, 0.003 wt % copper, and 0.001 wt % nickel.

Hereinafter reference will now be made in detail to various embodiments of the present invention, examples of which are illustrated in the accompanying drawings and described below. While the invention will be described in conjunction with exemplary embodiments, it will be understood that present description is not intended to limit the invention to those exemplary embodiments. On the contrary, the invention is intended to cover not only the exemplary embodiments, but also various alternatives, modifications, equivalents and other embodiments, which may be included within the spirit and scope of the invention as defined by the appended claims.

According to preferred embodiments of the present invention, the reason that the main components of the magnesium alloy of the present invention are added and the reason that the content of each component is limited are as follows.

(1) 1.00 to 3.00 wt % neodymium (Nd)

According to exemplary embodiments of the present invention, neodymium is added to form Mg₄₁Nd₅, which is a high-temperature stable phase precipitated along grain boundaries. Preferably, if the neodymium content is less than 1.00 wt %, the phase is not suitably formed, whereas, if it is more than 3.00 wt %, a coarse phase is suitably formed and the possibility of the formation of hot cracks during component manufacturing is increased, and thus the neodymium content is limited to 1.00 to 3.00 wt %.

(2) 2.00 to 6.00 wt % misch metal (Mm)

According to other further preferred embodiments, misch metal is added to form Mg₁₂Ce, which is a high-temperature stable phase precipitated along grain boundaries. Preferably, if the misch metal content is less than 2.00 wt %, the phase is not suitably formed, whereas, if it is more than 6.00 wt %, a coarse phase is suitably formed and the possibility of the formation of hot cracks during component manufacturing is increased, and thus the misch metal content is limited to 2.00 to 6.00 wt %.

(3) 0.10 to 1.00 wt % zinc (Zn)

According to still another further preferred embodiment, zinc is added to effectively form Mg₂YZn₆, Ca₂Mg₆Zn, and the like, which are high-temperature stable phases precipitated along grain boundaries. Preferably, if the zinc content is less than 0.10 wt %, the phase is not formed, whereas, if it is more than 1.00 wt %, elongation is suitably reduced due to the formation of a coarse phase, and thus it is preferred that the zinc content is limited to 0.10 to 1.00 wt %.

(4) 0.10 to 1.00 wt % zirconium (Zr)

According to another further embodiment of the present invention, zirconium is added to refine grains. Preferably, if the zirconium content is less than 0.10 wt %, the effect of zirconium is insignificant, whereas, if it is more than 1.00 wt %, a coarse Zn₂Zr phase is suitably formed, and thus it is preferred that the zirconium content is limited to 0.10 to 1.00 wt %.

(5) 0.01 to 0.50 wt % yttrium (Y)

In another further embodiment of the present invention, yttrium is added to form MgYZn₆, which is a high-temperature stable phase precipitated along grain boundaries. Preferably, if the yttrium content is less than 0.01 wt %, the phase is not formed, whereas, if it is more than 0.50 wt %, elongation is suitably reduced due to the formation of a coarse phase and the alloy price is increased, and thus the yttrium content is limited to 0.01 to 0.50 wt %.

(6) 0.01 to 0.10 wt % calcium (Ca)

In another exemplary embodiment, calcium is added to form Ca₂Mg₆Zn₃, which is a high-temperature stable phase precipitated along grain boundaries. Preferably, if the calcium content is less than 0.01 wt %, the phase is not formed, whereas, if it is more than 0.10 wt %, elongation is suitably reduced due to the formation of a coarse phase, and thus the calcium content is limited to 0.01 to 0.10 wt %.

(7) Less than 0.008 wt % silicon (Si)

(8) Less than 0.004 wt % iron (Fe)

(9) Less than 0.003 wt % copper (Cu)

(10) 0.001 wt % nickel (Ni)

In further exemplary embodiments of the present invention, silicon, iron, copper, and nickel are added as unavoidable impurities.

Next, the present invention will be described in more detail with reference to the following examples. However, the present invention is not limited to or by the following examples.

Examples 1 to 5 & Comparative Examples 1 to 5

In Examples 1 to 5, magnesium alloys composed of components as shown in the following table 1 were prepared according to the present invention, and specimens were prepared by a typical method of processing molten magnesium as a casting material. In Comparative Examples 1 to 5, conventional magnesium alloys composed of components as shown in the following table 1 were suitably prepared, and specimens were prepared in the same manner as Examples 1 to 5.

TABLE 1 unit: wt % Alloy Nd Zn Zr Y Ca Mm Mg Com- 2.7644 0.4208 0.3203 0.1369 0.0632 Bal. parative Example 1 Com- 1.3366 0.2440 0.3505 0.1853 0.0342 Bal. parative Example 2 Com- 1.6516 0.3057 0.3422 0.1196 0.0075 Bal. parative Example 3 Com- 2.1738 0.5472 0.3036 0.3522 0.0595 Bal. parative Example 4 Com- 2.4040 0.7598 0.3262 0.5122 0.0342 Bal. parative Example 5 Example 1 3.0 0.5 0.48 0.10 0.1 2.0 Bal. Example 2 2.5 0.5 0.48 0.10 0.1 3.0 Bal. Example 3 2.0 0.5 0.48 0.10 0.1 4.0 Bal. Example 4 2.5 0.5 0.48 0.10 0.1 4.0 Bal. Example 5 2.0 0.5 0.48 0.10 0.1 6.0 Bal.

Test Example 1

As described herein, an age-hardening behavior test on the specimens prepared according to Examples 1 to 5 and Comparative Examples 1 to 5 was conducted and, through the test, the heat-treatment temperature at which the maximum strength for each alloy could be obtained was suitably determined.

As a result, as shown in FIG. 1, the maximum strength for the alloys according to Comparative Examples 1 to 5 was suitably determined when the alloys were maintained at 230° C. for 20 min. and, as shown in FIG. 2, the maximum strength for the alloys according to Examples 1 to 5 was observed when the alloys were maintained at 250° C. for 20 min.

Test Example 2

According to another exemplary embodiment, the microstructure of the magnesium alloy specimen according to Example 1 was suitably observed by an electron microscope. As shown in FIG. 3, for example, MgNd and MgCe, which are high-temperature reinforcing phases which serve to maintain high-temperature resistant properties, were observed, which means that the magnesium alloy according to Example 1 is applicable to a cylinder block material of an engine which operates at an ambient temperature of 175 C.°.

Test Example 3

According to another exemplary embodiment, room-temperature yield strengths and high-temperature yield strengths (tensile strengths) of the magnesium alloy specimens were suitably prepared according to Examples 1 to 5 and Comparative Examples 1 to 5 were measured. As shown in FIG. 5, it can be seen that the high-temperature tensile strength (yield strength) of the alloy according to Example 1 was higher than that of the alloy according to Comparative Example 1, which means that the magnesium alloys according to the present invention can be used as a cylinder block material of an engine, instead of the conventional magnesium alloys.

Test Example 4

According to another exemplary embodiment, creep strain rates of the magnesium alloy specimens prepared according to Examples 1 to 5 and Comparative Examples 1 to 5 were measured. The results of creep-resistance test on the conventional magnesium alloys according to Comparative Examples 1 to 5 are shown in FIG. 6, and those of creep resistance test on the magnesium alloys of the present invention according to Examples 1 to 5 are shown in FIG. 7 and the following table 2.

TABLE 2 Alloys Creep strain rate (%) Comparative Example 1 0.112 Example 1 0.096 Example 2 0.115 Example 3 0.123 Example 4 0.108 Example 5 0.100

As shown in table 2, for example, it can be seen that the magnesium alloy of the present invention according to Example 1 had the lowest creep strain rate, i.e., the highest creep resistance properties.

Through the above-described Test Examples, it can be confirmed that the magnesium alloys according to the present invention has high-temperature tensile strength and creep resistance higher than the conventional magnesium alloys.

As described above, the present invention provides the following effects.

The present invention provides a heat-resistant magnesium alloy for gravity casting with high creep resistance, which can suitably improve the high-temperature tensile strength and creep resistance properties by controlling the content of the components of a conventional MRI202S alloy and, at the same time, by further adding misch metal as a rare earth metal.

Moreover, it is possible to suitably improve the hardness of the magnesium alloy by controlling the age hardening temperature and time.

The invention has been described in detail with reference to preferred embodiments thereof. However, it will be appreciated by those skilled in the art that changes may be made in these embodiments without departing from the principles and spirit of the invention, the scope of which is defined in the appended claims and their equivalents. 

1. A heat-resistant magnesium alloy for gravity casting with high creep resistance, the magnesium alloy containing magnesium as a main component, 1.00 to 3.00 wt % neodymium, 2.00 to 6.00 wt % misch metal, 0.10 to 1.00 wt % zinc, 0.10 to 1.00 wt % zirconium, 0.01 to 0.50 wt % yttrium, 0.01 to 0.10 wt % calcium, 0.008 wt % silicon, 0.004 wt % iron, 0.003 wt % copper, and 0.001 wt % nickel.
 2. The magnesium alloy of claim 1, wherein the magnesium alloy contains magnesium as a main component, 1.5 to 2.50 wt % neodymium, 2.00 to 4.00 wt % misch metal, 0.30 to 0.60 wt % zinc, 0.20 to 0.50 wt % zirconium, 0.09 to 0.20 wt % yttrium, 0.01 to 0.05 wt % calcium, 0.008 wt % silicon, 0.004 wt % iron, 0.003 wt % copper, and 0.001 wt % nickel. 